The current deserializer doesn't properly handle the '%' character based on whether it processes document directives or contents.
As the spec specifies here, if it processes document directives, '%' as the first character should be interpretted as the beginning of a directive; and if it processes document contents, '%' can be used as the first character of a scalar:

%YAML 1.2
---
%this key: is valid # the first '%' is not the beginning of a directive

So, this PR has modified the lexer so it can properly decide whether '%' is the beginning of a directive or the first character of a scalar.

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• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
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Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
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